In the case of delay-time methods of determining the filling level using guided electromagnetic measuring signals, different types of waveguides are used. It has become known to use metal rods or tubes as waveguides. Because of their smooth surface, tubes or metal rods are distinguished by the advantages mentioned below:    low attenuation of the high-frequency measuring signals;    reduced tendency for the formation of deposits;    low tensile forces in bulk materials;
Since there is in any case a small flexibility, the diameter of the rod can be chosen to be large without further restricting functionality. This leads to a further reduction in the attenuation of the highfrequency measuring signals, since the high-frequency surface currents can flow in an enlarged surface area.
Furthermore, the undesired influence of adhering product (absent echoes, attenuation of the measuring signals, measuring errors due to changed propagation velocity of the measuring signals) is reduced as a result, since the radial extent of the field is increased with a larger waveguide diameter. As a result, the direct surroundings of the waveguide play a more minor role.
However, the use of metal rods or tubes for delay-time measurements with guided high-frequency measuring signals also has disadvantages. These are listed below:    more difficult transport, if the tubes or metal rods are delivered in one piece;    if the rod/tube is made such that it can be dismantled for easy transport (for example screwed with threaded bolts), the flexural and/or tensile load-bearing capacity is reduced;    in bulk materials, there is the risk of an irreversible deformation of the rod/tube caused by the effect of lateral forces    in bulk materials, high torques are transferred, which may lead to damage of the coupling-in element itself or to the silo;    it is scarcely possible to install the rigid sensor in partly filled solid-material silos.
In addition, it has become known to use bare wire cables, known as 6×19+SEL wire cables, as waveguides for filling-level measurement with guided high-frequency measuring signals. These wire cables, which are twisted stranded wires, have the following advantages:    high flexibility, allowing the wire cables to be transported in the rolled-up state;    uncomplicated installation of the same in partly filled solid-material silos;    only tensile forces are transferred, causing at most small torques to act on the coupling-in element.
However, the bare, twisted stranded wires also have disadvantages, which are listed below and make the use of this type of waveguides appear in an unfavorable light for the use aimed for by the invention:    high attenuation of the high-frequency signals, since the current flowing in the longitudinal direction must pass very many contact points between the individual wires (this is problematical in particular at relatively high frequencies of several GHz, since here the attenuation is in any case already relatively high due to the skin effect);    at frequencies of several GHz, the attenuation additionally depends on the tensile loading on the cable: acceptable attenuation values are only achieved with high tensile forces of several 1000 N and more. This is presumably to do with the fact that only under tension is there intimate contact of the individual wires of the cable, and consequently a small contact resistance. However, it is precisely lightweight products with small dielectric constants (plastics, powdered media), with which the useful signal is in any case small owing to the low reflection at the surface, that exert low tensile forces on the cable. At low filling levels, these forces are at a minimum, while at the same time the path of the measuring signal on the waveguide is at a maximum, and consequently disturbs the attenuation most.    low resistance to abrasion (for example in the case of sand or corundum as the filled product), causing the fine individual wires to be rubbed through at the surface after only a short time. Splitting-open wires are a consequence of abrasion, with the effect that the loading at this point is increased further.    high tensile forces in solid materials as a result of the roughened surface;    for the same reason, there is a strong tendency for the formation of deposits;    low resistance to twisting, which can easily lead to destruction of the cable. The structure of the cable with only one outer layer can be seen as the reason for this.
Furthermore, it has become known to use 6×19+SEL wire cables with plastic coating as waveguides for filling-level measurement with guided high-frequency measuring signals. In addition to the advantages shown by bare wire cables, the following advantages also come into effect here:    better resistance to abrasion, since the tough but pliable plastic does not rub through as quickly—the smooth surface does not offer any points where coarser bulk materials can act abrasively;    in the case of some plastics (PTFE), there is a reduced tendency for deposits to form because of the low adhesion between plastic and the filled product; the formation of deposits is always very low in the case of all plastics because of the smooth surface.
However, the coated wire cables also have the following disadvantages:    low temperature and aging resistance;    restricted suitability for use in explosive atmospheres, since the plastic can be electrostatically charged and consequently represents a possible source of ignition;    poor tensile load-bearing capacity in relation to tensile forces occurring. The former is dictated by the (small) diameter of the metal core, the latter are proportional to the surface area of the cable and consequently to the (large) diameter of the plastic sheath    very high attenuation of the measuring signals, since the dielectric losses add to the conductor losses of uncoated cables.
Taking the prior art as a starting point, the invention is based on the object of optimizing the waveguide.